The present invention relates to a torque detector for detecting a rotating torque exerted on a steering member for steering in, for example, an electric power steering apparatus, and a method of manufacturing a housing of the torque detector.
There is an electric power steering apparatus in which a motor for assisting a steering in accordance with the rotating operation of a steering member for steering (for instance, a steering wheel) is driven to transmit the torque of the motor to a steering mechanism and assist the steering. In the power steering apparatus, the rotating torque exerted on the steering member needs to be detected to use the rotating torque for controlling to drive the motor for assisting the steering. For detecting the rotating torque, a torque detector formed halfway a rotating shaft (column shaft) connecting the steering member to the steering mechanism has been hitherto employed.
In the torque detector, the column shaft is divided into a first shaft and a second shaft connected in the coaxial direction through a torsion bar having a small diameter. When the steering member for steering is rotated, a relative angular displacement is generated between the first and second shafts accompanied by the displacement of the torsion bar due to the action of the rotating torque so that the rotating torque is detected through the relative angular displacement.
The relative angular displacement can be detected with such a structure as described below (for instance, see Patent Document 1). For instance, detecting rings are respectively externally fitted and fixed to parts near the connecting part of both first and the second shafts. A sensor coil (sensor part) is internally fitted and held so as to enclose the opposed parts of the detecting rings in a sensor holding hole provided inside a tubular housing for supporting the column shaft to form a magnetic by supplying coil current to the sensor coil. Thus, the change of the opposed relation of the respective detecting rings corresponding to the relative angular displacement between the first and second shafts due to the operation of the rotating torque is taken out as the change of impedance of the sensor coil in accordance with the increase or decrease of the magnetic resistance of the magnetic circuit through a circuit board attached to the inner part of a board chamber which is isolated from the sensor holding hole inside the housing (for instance, see Patent Document 1).
The board chamber to which the circuit board is attached communicates with the inner part of the sensor holding hole through a lead hole passing through a partition wall between the sensor holding hole and the board chamber. The sensor coil is connected to the circuit board through a connecting lead extended from the sensor coil, inserted into the lead hole and guided to the board chamber.                Patent Document 1        JP-B-7-21433        
The attached of the torque detector constructed as described above is realized in such a procedure as described below. The sensor coil as the sensor part is fitted to the inner part of the sensor holding hole and fixed and held at a prescribed position. The circuit board is attached to the board chamber inside the housing. The circuit board is connected to the sensor coil through the connecting lead inserted into the lead hole as described above.
However, in the above-described attaching procedure, the sensor coil is fitted to the inner part of the sensor holding hole via an opening part at one side in the longitudinal direction. The circuit board is attached to the inner part of the board chamber from outside in the diametrical direction of the housing. These operations are carried out in separate processes. In addition thereto, during these operations, an operation for connecting the connecting lead inserted into the lead hole to the circuit board has to be done and a large number of attaching processes are inconveniently required.
The housing to which the sensor coil and the circuit board are attached is ordinarily made of aluminum casting for the purpose of achieving a lightweight. The after-treatment of the sensor holding hole for internally fitting and holding the sensor coil, the board chamber to which the circuit board is attached and the lead hole through which both the members communicate with each other is necessary. Many processes are undesirably required to work them. Further, the output of the torque detector taken out from the sensor coil through the circuit board is affected by external electromagnetic noise generated in an engine, devices mounted on a vehicle or the like. Thus, there is a fear that detection accuracy may be possibly deteriorated.
The torque detector used for detecting the rotating torque exerted on the steering member in the electric power steering apparatus is not limited to the above-described structure including the detecting rings externally fitted and fixed to the column shaft and the sensor coil internally fitted and held in the sensor holding hole of the housing. The torque detector is put to practical use in various kinds of structures. In any of the structures, the sensor part held in the sensor holding hole of the housing is connected to the circuit board attached to the inner part of the board chamber isolated from the sensor holding hole. The output of the sensor part is taken out through the circuit board. The output of the sensor part held in the sensor holding hole of the housing is taken out through the circuit board attached to the outer side of the housing. Accordingly, it is an important purpose to easily attach the sensor part and the circuit board to the housing, reduce the number of working processes of the housing for attaching them and prevent the deterioration of detection accuracy due to the influence of electromagnetic noise.